This invention relates to a sheet paper feeder of the type in which a flat paper or, sheet such as printing paper or an interleaved paper, is held between sheet feeding rollers and sheet holddown wheels, and is then fed toward a sheet feeding cylinder or a sheet delivery apparatus.
In a sheet-feed offset printing press, a sheet paper feeder is provided so that, after printing papers successively fed from an automatic sheet feeder are temporarily stopped by the combination of front register lays and side register lays regulating the register of the printing papers, the printing papers are accelerated up to a speed equal to the peripheral speed of an impression cylinder and are then delivered with accurate timing toward a gripper of the impression cylinder.
Further, in the case of, for example, intaglio printing where printing papers are printed with an ink film having a relatively large film thickness, stacking of the printing papers in tiers on a pile board tends to give rise to an undesirable offset.
In order to avoid such an undesirable offset, a method is commonly employed in which an interleaving paper is interposed between the printing papers to be vertically stacked in tiers on the pile board. In this case, it is necessary to feed the interleaving papers with accurate timing toward a gripper of a sheet delivery apparatus so as to interpose the interleaving paper between the printing papers vertically stacked in tiers on the pile board. In this case too, a sheet paper feeder of the kind described above is used.
As one form of such a sheet paper feeder, a feed-roll type is known in which a sheet, such as printing paper or interleaved paper, is held between rotating sheet feeding rollers and holddown wheels disposed opposite to the feeding rollers and is fed under acceleration toward a feeding cylinder or a sheet delivery apparatus.
In a prior art sheet paper feeder of the feed-roll type, the sheet feeding rollers are continuously driven to rotate at a peripheral speed corresponding generally to the peripheral speed of an impression cylinder, or that of delivery chains of a sheet delivery apparatus. The sheet holddown wheels disposed opposite to the sheet feeding rollers are reciprocated toward and away from the sheet feeding rollers. As soon as the holddown wheels are moved toward the sheet feeding rollers, and a sheet is held between the sheet feeding rollers and the sheet holddown wheels, the sheet is fed toward the impression cylinder or the delivery chains of the sheet delivery apparatus.
However, in the prior art sheet paper feeder having the sheet feeding rollers and the sheet holddown wheels described above, slip tends to occur between a sheet and the sheet feeding rollers when the sheet feed rate exceeds 100 sheets per minute or when the sheet is, for example, a coated sheet having a slippery surface.
Thus, the prior art sheet paper feeder has the disadvantages that the occurrence of slip between the sheet and the sheet feeding rollers leads to delayed timing of sheet feeding, with the result that a wasted paper is delivered due to deviation of the sheet from its predetermined printing position, or, failure of proper gripping of the sheet by the gripper gives rise to undesirable stoppage of the operation of the printing press.
As described above, in the prior art sheet paper feeder, a printing paper or an interleaving paper is temporarily maintained in a stationary state after it is registered and is then instantaneously accelerated up to the speed equal to the peripheral speed of the impression cylinder or the delivery chains of the sheet delivery apparatus in the printing press.